The equivalent is false alarm:
“The doctors came flying in the room, a whole team of doctors, and said, ‘You’re not going to believe this. We have a liver for you. You’re getting a liver transplant in just a few hours,’” she said. Unfortunately, it was a false alarm. The doctors said it was difficult to find the perfect liver for Shiroda, who is blood type A-positive. —“Family gives special thanks to organ donation”, WAFB.
[T]he Princeton Dental Resource Center said research showed chocolate could fight cavity-causing plaque … [u]nfortunately, it was a false alarm, one that demonstrates the pitfalls of getting advice on dental health from a candy manufacturer. —“Find Who’s Behind Dubious Declarations”, SunSentinel.
@RealMichelleT tweeting about Mud Wrestling and got all excited. Unfortunately, it was a false alarm. —James D (@TuckingFypos) on Twitter
As an idiomatic expression, false alarm refers to a signal which is afterwards discovered to be false: originally a literal false alarm (such as a fire alarm which sounds in the absence of fire, or a false report of fire to the city), but now also a metaphor in which the signal is not a literal alarm and not necessarily even an alarming (scary) thing.
The examples above were found by googling [
"unfortunately it was a false alarm" ]. A Wordnik search for false-alarm returns additional metaphorical examples of “alarms”, some with no negative association, such as:
Gould is referring, of course, to the whipped-up brouhaha surrounding his own three false-alarm “revolutions” in Darwinism: exaptation, punctuated equilibrium, and, most recently, species selectionism.
In the example above, the false alarm is positive: revolutions in the scientific understanding of how species evolve.
For an idiomatic expression such as false alarm, it can be misleading to look up the individual words instead of the expression. Consider instead the attestations given in the following dictionaries:
A signal or warning that is groundless —“false alarm”, sense 2 in American Heritage Dictionary of the English Language, Fourth Edition [sense 2]
[S]omething that excites unfounded alarm or expectation —“false alarm”, sense 2 at Dictionary.com